PROJECT SUMMARY The kidney plays a critical role in defining the pharmacokinetics (PK) of cationic drugs, and these ?organic cations? (OCs) include about 40% of prescribed drugs. Renal OC secretion involves three transport processes: an Organic Cation Transporter (i.e., OCT2), and two Multidrug and Toxin Extruders (MATE1 and MATE2-K). Understanding the influence of these transporters on cationic drug PK relies on use of ?physiologically-based pharmacokinetic? (PBPK) models that link in vitro measurements of transport kinetics (determined in cultured cell systems) to the in vivo activity of these processes in human kidney, i.e., ?in vitro-in vivo extrapolation? (IVIVE). PBPK modeling is used by the pharmaceutical industry, and information derived from these models is part of regulatory submissions. But to date, a lack of accurate data for IVIVE for cationic drug transport has confounded application of PBPK to renal OC transport. Consequently, modeling of drug transport is restricted to retrospective efforts that rely on (often) arbitrary manipulation of in vitro transport data and use of ?relative expression factors? that have no mechanistic basis. The goal of this proposal is improving PBPK modeling of renal secretion of cationic drugs by increasing the accuracy of measurement of in vitro transport kinetics; and measurement of transporter abundance in in vitro cultured cell models and (in vivo) human renal tissue. We propose Five Aims to achieve this goal. Aim 1 employs cultured cell models to determine (i) physiologically accurate Kt and Jmax values for OCT2, MATE1 and MATE2-K-mediated transport of a suite of substrates; and (ii) the absolute abundance of these transporters (using both existing and novel tools applied to quantitative Western blotting). Aim 2 uses these tools to determine the amount and distribution of these transporters in human renal tissue, and demonstrate the use of that information in PBPK modeling. Aim 3 addresses the lack of data on the potential influence of MATE2-K to renal drug secretion by establishing its selectivity vs. that of MATE1, and developing models of ligand interaction made accessible (via internet and mobile apps) so that other workers may interrogate them directly. Aim 4 applies a novel technology that is applicable to any transport process, for making isoform-specific high affinity nanobody-based peptide binders (IS-Nbs) to study the expression, distribution and contribution of specific transporters to drug clearance. We will develop selective binders of OCT2, MATE1, and MATE2-K that will serve as tools to establish the relative roles of these processes in renal drug transport. Aim 5 uses these tools to conduct a proof-of-concept study on the in vivo use of IS-Nbs to define the contribution of individual transporters to the complex process of renal OC secretion. In summary, we will apply existing and novel biological reagents capable of studying many facets of cationic drug transport to assist using PBPK for the prediction and preemption of adverse drug reactions; and in drug development; while developing methods that should be applicable to other drug transporters.